Dario Argento leaves a distinct and bloody impression with this Italian horror film that took the slasher genre to graphic new limits at the time of its release. Novelist Peter Neal (Anthony Franciosa) jets into Rome to promote his new book. Simultaneously, a killer obsessed with Neal begins a brutal series of murders that are followed by cryptic notes to the author. Inspector Germani (Giuliano Gemma) questions Neal, who then begins his own investigation into the bizarre case with the help of his assistant, Anne (Daria Nicolodi), and local youth Gianni (Christian Borromeo). Neal and Gianni follow leads to the home of a TV talk-show host (John Steiner), who is axed to death in front of Gianni while Neal is knocked unconscious.

As they close in on the killer, flashbacks show the killer's murderous beginnings and an obsession with red shoes. Meanwhile, Neal's publicist, Bullmer (John Saxon), is revealed to be having an affair with the author's ex-lover, Jane (Veronica Lario), making them both potential suspects. Inspector Germani insists that Neal leave town, but even when he does, the killer strikes again, knifing Bullmer in broad daylight. At the same time, Gianni returns to the home of the dead talk-show host and recalls an important detail about the murder. However, he is strangled before he can tell anyone. At her apartment, Jane is brutally slain just as Inspector Germani arrives to discover the murderer's identity, along with the shocking, twist-filled truth behind the entire case.